<p><b>Abstract</b>—Resolving domain incompatibility among independently developed databases often involves uncertain information. DeMichiel [<ref rid="bibk07071" type="bib">1</ref>] showed that uncertain information can be generated by the mapping of conflicting attributes to a common domain, based on some domain knowledge. In this paper, we show that uncertain information can also arise when the database integration process requires information not directly represented in the component databases, but can be obtained through some summary of data. We therefore propose an extended relational model based on <it>Dempster-Shafer theory of evidence</it> [<ref rid="bibk07072" type="bib">2</ref>] to incorporate such uncertain knowledge about the source databases. The extended relation uses evidence sets to represent uncertainty in information, which allow probabilities to be attached to subsets of possible domain values. We also develop a full set of extended relational operations over the extended relations. In particular, an extended union operation has been formalized to combine two extended relations using <it>Dempster's rule of combination</it>. The <lit>closure</lit> and <lit>boundedness</lit> properties of our proposed extended operations are formulated. We also illustrate the use of extended operations by some query examples.</p>